ITV's critical end-of-year TV trading season, when deals to secure more than £1.5bn from advertisers for 2012 will be concluded, could to be thrown into turmoil because of a dispute with Richard Desmond's Northern & Shell over a non-compete clause for a senior executive.

Kelly Williams, ITV's new sales director, is due to join the company in August. However, Williams' former employer, Northern & Shell, is understood to be trying to keep him to a 12-month non-compete clause, which would prevent him from leading ITV's 2012 airtime sales negotiations in the autumn.

While this was seen as a brazen move the timing was considered astute as the critical period for TV ad sales comes in the final quarter of each year. This is when deals for more than 80% of the £3.5bn spent annually on UK TV advertising are negotiated between the TV sales houses of the commercial broadcasters and media agencies on behalf of their clients.

However, it has emerged that Northern & Shell, the parent company of Channel 5, is standing firm on a one-year non-compete clause, effectively barring Williams from dealing with clients until January.

"Kelly Williams is starting at ITV on 1 August but under [the] non-compete [deal] he can't speak to any Channel 5 competitors or customers until January," said a source at Northern & Shell.

This would be a blow to ITV, which is already heading into a tough summer with advertising expected to be down 9% in May and 20% in June because in the same period last year it benefited from a surge in advertising due to the World Cup.

One senior industry executive who has previously been involved in a similar tussle over a non-compete clause said that if the matter went to court Northern & Shell is unlikely to win – but it would be a pyrrhic victory for ITV.

"These things [a non-compete] are difficult to enforce and ultimately a court will probably say it is a restriction of trade but, and it is a big but, to get a court to say it is invalid you have to go through a very expensive, very time-consuming and very painful process," said the executive. "How tough rather depends on who the [previous] employer is and how tough they want to get about it."

An ITV spokesperson said: "ITV can confirm that Kelly Williams will be joining as director of TV Sales on 1 August 2011."

Northern & Shell declined to comment.

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